Laser range finding device and distance measurement  method thereof

ABSTRACT

A distance measurement method for use in a laser range finding device to measure a distance between the laser range finding device and a target is disclosed. The method comprises the following steps. A laser signal is sent to the target in a first time point. A reflected laser signal reflected by the target is then received. A digital signal having a plurality of signal values ranging from 0 to N is obtained by sampling the reflected laser signal with a sampling signal, wherein N is an integer larger than two. A maximum signal value among the signal values is obtained. The distance is calculated according to the first time point and a second time point where the maximum signal value is generated.

This application is a continuation application of U.S. patentapplication Ser. No. 12/885,852, “Laser Range Finding Device & DistanceMeasurement Method Thereof,” filed on Sep. 20, 2010, by Luo, et al.,which is a continuation application of U.S. patent application Ser. No.12/047,350, “Laser Range Finding Device & Distance Measurement MethodThereof,” filed on Mar. 13, 2008, by Luo, et al., Now U.S. Pat. No.7,800,737. which claims priority to Taiwan Patent Application No.96122879, filed Jun. 25, 2007, all of which are incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a laser range finding device and distancemeasurement method thereof, and more precisely, to a laser range findingdevice and distance measurement method for measuring a distance betweena laser range finding device and a target using time of flight (TOF) oflaser signals.

2. Description of the Related Art

Generally, distance measurement methods using laser signals can beclassified into three types: phase, interference and pulse feedbacktypes. For a laser range measurement with a pulse type, a distancebetween a laser range finding device and a target is calculated bydirectly measuring time of flight of the laser light pulse signal. Whena laser diode is trigged by a driving circuit, it generates a startpulse as a starting signal of a time distance measurement system tostart counting time. Then, after a reflected laser signal has beenreceived and amplified, the laser diode generates a stop pulse to stopthe time count. A counting circuit measures a time difference betweenthe start pulse and the stop pulse to calculate and obtain a time offlight of the transmitted laser pulse signal from the target to thelaser range finding device, thereby obtaining the distance between thelaser range finding device and the target using the calculated time offlight.

However, when performing a long distance measurement (i.e., the targetis far away from the laser range finding device), the signal strength ofthe reflected signal received by the laser range finding device may bevery weak, such that the signal strength of the received signal is onlya little larger than that of a noise signal. In this case, noise signalvalues may be erroneous determined as the target signal if apredetermined threshold level is set too small or the weak signalreflected from the target will not be detected if the predeterminedthreshold level is set too large.

It is therefore desired to provide methods and apparatuses for improvingthe ability of the distance measured for the laser range finding device.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the invention provides and discloses a distancemeasurement method for use in a laser range finding device to measure adistance between the laser range finding device and a target. Thedistance measurement method comprises the following steps. First, alaser signal to the target is transmitted in a first time point. Then, areflected laser signal reflected by the target is received. Next, adigital signal having a plurality of signal values is obtained bysampling the reflected laser signal with a sampling signal, wherein thesignal values range from 0 to N where N is a positive integer greaterthan two. A maximum signal value among the signal values of the digitalsignal is then found. Finally, the distance between the laser rangefinding device and the target is calculated according to the first timepoint and a second time point corresponding to the maximum signal value.

An embodiment of the invention also provides and discloses a laser rangefinding device for measuring a distance between the laser range findingdevice and a target. The laser range finding device at least comprises atransmitter, a receiver, an analog-to-digital converter and a systemunit. The transmitter transmits a laser signal to the target in a firsttime point. The receiver receives a reflected laser signal reflected bythe target. The analog-to-digital converter samples the reflected lasersignal with a sampling signal to obtain a digital signal having aplurality of signal values, wherein the signal values range from 0 to Nwhere N is a positive integer greater than two. The system unit finds amaximum signal value among the signal values of the digital signal andcalculates the distance between the laser range finding device and thetarget according to the first time point and a second time point thatgenerates the maximum signal value.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with reference to the accompanyingdrawings, wherein:

FIG. 1 schematically shows a block diagram of an embodiment of a laserrange finding device;

FIG. 2 is a block diagram illustrating an embodiment of a laser rangefinding device according to the invention;

FIG. 3 is a signal waveform diagram;

FIG. 4 is a flowchart of an embodiment of a distance measurement methodaccording to the invention;

FIG. 5 is a flowchart of another embodiment of a distance measurementmethod according to the invention;

FIG. 6 schematically shows an embodiment of a mapping table according tothe invention; and

FIG. 7 schematically shows an embodiment of a digital signal accordingto the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

The invention is now described with reference to FIGS. 1 through 7,which generally relate to a laser range finding devices and distancemeasurement methods thereof. In the following detailed description,reference is made to the accompanying drawings which form a part hereof,shown by way of illustration of specific embodiments. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of theinvention. The following detailed description is, therefore, not to betaken in a limiting sense. It should be understood that many of theelements described and illustrated throughout the specification arefunctional in nature and may be embodied in one or more physicalentities or may take other forms beyond those described or depicted.

The embodiments of the invention provide a laser range finding deviceand distance measurement method thereof so as to obtain a distancebetween the laser range finding device and a target. In one embodiment,a laser range finding device is provided. The laser range finding deviceperforms a digitalization operation for a received reflected lasersignal by an analog-to-digital converter (ADC) so as to generate adigital signal comprising a multitude of signal values. Given thegenerated digital signal that comprises a multitude of signal values,original waveform and amplitude of the reflected laser signal can bereserved so that the noise signal and the target signal can bedistinguished, thus, improving the ability to guard against noiseinterference, to perform long distance measurements, and to shorten thetime for distance measurements.

In an embodiment of a distance method according to the invention, thereflected laser signal is utilized to obtain a digital signal comprisinga plurality of signal values, and a maximum signal value among all ofthe signal values of the digital signal is then found. Next, a time offlight of the laser signal is calculated using the time point whichgenerates the maximum signal value thereby obtaining the distancebetween the laser range finding device and the target, wherein the timeof flight of the laser signal indicates a time difference from a firsttime point that the laser signal is transmitted to a second time pointthat a reflected laser signal is received.

FIG. 1 schematically shows a block diagram of an embodiment of a laserrange finding device 100. As shown in FIG. 1, a transmitter in the laserrange finding device 100 transmits a laser signal to a target 110 andthen a receiver receives the reflected laser signal. The signal valuesof the reflected laser signal are converted to a value of one or zero bya comparator, processed by a signal processing unit to find out the timepoint that a target signal is received so as to determine a distance tothe target.

The comparator compares the inputted signal value to a threshold levelto determine an output value. A value “1” is outputted when the inputtedsignal value is larger than or equal to the threshold level, otherwise avalue “0” is outputted. When the target distance is extended, the signalstrength of the reflected laser signal received by the receiver willbecome relatively weak, resulting in increased interference by noisesignals. As a result, for finding the target signal, the threshold levelhas to be set very small such that noise signals may be erroneouslydetermined as the target signal should the signal value of the noisesignal exceed the threshold level. In this case, the target signal maynot be actually recognized thereby extending the time needed fordistance measurement.

FIG. 2 is a block diagram illustrating an embodiment of a laser rangefinding device 200 according to the invention, for measuring thedistance therefrom to a target 260. The laser range finding device 200comprises a transmitter 210, a receiver 220, a analog-to-digitalconverter (ADC) 230, a system on chip (SOC) 240 and a displaying unit250. The transmitter 210 transmits a laser signal L1 to the target 260for measuring the distance. The transmitter 210 further comprises apulsed laser diode 212 and a driving circuit 240, wherein the drivingcircuit 240 controls the laser diode 212 to transmit the laser signal tothe target 260.

The receiver 220 receives a reflected laser signal L2 reflected by thetarget 260, which comprises an avalanche photodiode 222 for receivingthe reflected laser signal reflected by the target, a signal amplifier224 for amplifying the reflected laser signal received by the avalanchephotodiode 222, and a filter circuit 226 for performing a noisefiltering to the amplified reflected laser signal. The analog-to-digitalconverter 230 is coupled to the receiver 220, performing a signaldigitalization to the reflected laser signal from the receiver 220 so asto obtain a digital signal having a plurality of signal values. It is tobe understood that the digital signal has a specific waveform as same asthat of the reflected laser signal and all of the signal values rangefrom 0 to N where N is a positive integer greater than two. Referring toFIG. 3, a signal waveform diagram is illustrated. In FIG. 3, signals S1,S2 and S3 represent waveforms of the received reflected laser signal L2,a digital signal that has been digitalized by the comparator of thelaser range finding device 100 in FIG. 1, and a digital signal that hasbeen digitalized by the analog-to-digital converter 230 of the laserrange finding device 200 in FIG. 2, respectively. It is assumed that, inthis embodiment, the analog-to-digital converter 230 is an eight bitsanalog-to-digital converter (ADC) so that the reflected signal isconverted to a digital signal that has a plurality of signal valuesranging from 0 to 255. As shown in FIG. 3, signal values of threesampling points P1, P2 and P3 on the reflected laser signal S1 are allconverted to “1” on the digital signal S2 while those values of thethree sampling points P1, P2 and P3 on the reflected laser signal S1 arerespectively converted to “100”, “250” and “105” on the digital signalS3. It is observed that the output of the digital signal S2 forming awaveform which is a square wave due to each output of the digital signalS2 is determined by comparing the inputted signal value to a thresholdlevel, not representing that the sampling point P2 has a maximum peakvalue or a maximum signal value. The signal values of digital signal S3,however, may be quantified as various signal values, not onlyrepresenting that the sampling point P2 has a maximum peak value or amaximum signal value but also keeping a waveform as same as that of theoriginal signal S1.

The system on chip (SOC) 240 comprises a control logic unit 242, amemory unit 244 and a processing unit 246. The control logic unit 242 iscoupled to the processing unit 214, the transmitter 210 and theanalog-to-digital converter 230, driving the transmitter 210 to transmitthe laser signal to the target 260 and activating the analog-to-digitalconverter starting signal conversion when it receives a trigger signalfrom the processing unit 246. The memory unit 244 stores a digitalsignal converted by the analog-to-digital converter 230. Then, theprocessing unit 246 reads the stored digital signal from the memory unit244 and finds a maximum signal value among all of the signal values ofthe digital signal to calculate a time of flight of the laser signal soas to obtain the distance between the laser range finding device 200 andthe target 260.

FIG. 4 is a flowchart 400 of an embodiment of a distance measurementmethod according to the invention. In this embodiment, refer to bothFIGS. 2 and 4. In step S410, a laser signal is transmitted to the target260 in a first time point T1. The time point T1 is the starting time thelaser signal begins transmission. Meanwhile, the control logic unit 244receives a trigger signal from the processing unit 246 and prepares toperform the distance measurement. Accordingly, during the first timepoint, the control logic unit 242 drives the driving circuit 214 of thetransmitter 210 to control the laser diode 212 to transmit the lasersignal L1 to the target 260 and activates the analog-to-digitalconverter 230 starting signal conversion.

In step S420, a reflected laser signal L2 reflected by the target 260 isthen received by the avalanche photodiode 222 of the receiver 220,amplified by the signal amplifier 224, filtered for noise from theamplified reflected laser signal by the filter circuit 226 to achieve afiltered result, and outputted (e.g, signal S1 shown in FIG. 3) to theanalog-to-digital converter 230. In step S430, the analog-to-digitalconverter 230 converts the reflected laser signal from the receiver 220to a digital signal so as to generate a digital signal having aplurality of signal values.

For example, referring to FIG. 3, assume that the analog-to-digitalconverter 230 is an eight bits analog-to-digital converter, thereflected laser signal S1 will be converted to the digital signal S3having a plurality of signal values ranging from 0 to 255, in which thewaveform of the digital signal S3 is as same as that of the signal S3.After the digital signal has been generated by the analog-to-digitalconverter 230, the digital signal is stored in the memory unit 244.

In step S440, the processing unit 246 reads the stored digital signalfrom the memory unit 244 and finds a maximum signal value among all ofthe signal values of the digital signal. In step S450, the processingunit 246 calculates a time of flight of the laser signal using the firsttime point T1 and a second time point T2 that generated the maximumsignal value. In this embodiment, the time of flight is defined as thetime that the laser signal leaves the laser diode 212 to the time it isreceived by the analog-to-digital converter 230. In this case, the timeof flight T=T2−T1.

In step S460, the distance between the laser range finding device 200and the target 260 is determined by multiplying the calculated time offlight T by the speed of e light. In this case, the signal values arewithin a large range (i.e. 0 to N, N>2), depending on the bit number ofthe used analog-to-digital converter, such that a difference between thenoise signal value and the target signal value is larger than one. As aresult, the target signal can be easily and actually located and found,shortening the time needed for distance measurement.

For ensuring the accuracy of the maximum signal value time point, in oneembodiment, a threshold level is further applied to help determinationof the maximum signal value (i.e. target signal).

FIG. 5 is a flowchart 500 of another embodiment of a distancemeasurement method according to the invention.

As shown in FIG. 5, in step S510, a threshold level corresponding toenvironmental parameters (such as weather, strength of sun light andnoise of the test environment) is obtained. The environmentalparameters, such as strength of sun light, can be repeatedly tested inadvance to acquire a group of corresponding statistical data to serve asthreshold levels later. To acquire the group of statistical datacorresponding to the environmental parameters, a number of laser signalsare transmitted to the sky and then reflected laser signals are receivedby the receiver of the laser range finding device as sampled signals.Next, the sampled signals are analyzed to find a relationship of theaverage signal value AV which is an average value of those sampledsignal values and a difference Δ V of the found maximum signal value anda relative maximum signal value found in an area outside of the maximumsignal value within a predetermined range. Once the relationship hasbeen found, the threshold level can be set as a value that exceeds amaximum value of the difference Δ V. For example, for the average signalvalue AV ranging from 18000 to 19000, the measured possible range of thedifference Δ V is from 0 to 40, so the threshold level can be set as 42,for example, if the sampled average signal value AV is a value thatblows 19000.

This relationship of the average signal value AV and the correspondingthreshold level can be acquired in advance and stored in a mapping tableof the memory unit. FIG. 6 schematically shows an embodiment of amapping table 600 according to the invention. The mapping table 600 hasan average value AV field representing the average signal value AV and athreshold level field in which each average value AV field correspondsto a threshold level. For example, by looking up the mapping table, thethreshold level is set as 42 if the average value AV is 18500, while thethreshold level is set as 62 if the average value AV is 19501.

Accordingly, in step S520, a second larger signal value located in anarea outside of the maximum signal value within a predetermined range isfound. Note that the predetermined range can be freely adjusted and itis normally a value around the maximum signal value so as to preventperforming erroneous comparisons. For example, in this embodiment, arelative maximum value (i.e. the second larger signal value) is definedas a maximum signal value acquired in an area outside of a time intervalthat utilizes the second time point as a reference central point andranged from a time point that is one time unit prior to the second timepoint to a time point that is one time unit subsequent to the secondtime point, in which the time unit can be one cycle of the samplingsignal of the reflected laser signal. The predetermined range can bedefined as, for example, a time interval that utilizes the maximumsignal value as a reference central point within three yards, but is notlimited thereto.

FIG. 7 schematically shows an embodiment of a digital signal accordingto the invention. As shown in FIG. 7, the signal value at a point V1 isfound as the maximum signal value, and a relative maximum signal value(i.e. the signal value at a point V2) is thus, found in an area outsideof the signal value of the point V1 within a predetermined range.

Next, in step S530, it is determined that whether a difference Δ V (i.e.V1−V2) between the maximum signal value and the relative maximum signalvalue is not less than a selected threshold level. If so (Yes in stepS530), the found maximum signal value is determined as the target signalthereby the second time point that the maximum signal value has occurredis recognized as being correct (step S540). If not (No in step S530), itimplies that the found maximum signal value may be a noise signal,therefore, the laser signal is re-transmitted to perform anothermeasurement (step S550).

In addition, referring to FIG. 7, a maximum value would be found afterthe time point that the maximum signal value has occurred (as the signalvalue at time point T3 shown in FIG. 7) due to the property of thehardware devices (such as the analog filter). Such property of thehardware devices, therefore, may be used to help determination of thetarget signal.

In one embodiment, a relative minimum signal value can be acquired afterthe maximum signal value has occurred, and then it is determined thatwhether a difference Δ V2 (i.e. V1−V3) between the maximum signal valueand the relative minimum signal value is not less than a secondthreshold level. If so, the found maximum signal value is determined asthe target signal thereby the second time point that the maximum signalvalue has occurred is recognized as being correct. If not, it impliesthat the found maximum signal value may be a noise signal, therefore,the laser signal is re-transmitted to perform another measurement. Inanother embodiment, when the difference Δ V between the maximum signalvalue and the second larger signal value (the relative maximum signalvalue) is less than the selected threshold level (No in step S530), itis further determined that whether the difference Δ V2 (i.e. V1−V3)between the maximum signal value and the relative minimum signal valueis not less than a second threshold level. If so, the found maximumsignal value is determined as the target signal and thereby the secondtime point that the maximum signal value has occurred is recognized asbeing correct. If not, it implies that the found maximum signal valuemay be a noise signal, therefore, the laser signal is re-transmitted toperform another measurement.

It is to be noted that, as would be apparent to those skilled in theart, the although only one set of laser signals are illustrated, it isalso possible to transmit a number of sets of laser signals for distancemeasurement. In this case, the receiver is capable of receiving a numberof reflected laser signals corresponding to the transmitted ones andaccumulating or averaging these received reflected laser signals so asto perform further processes using the accumulated or averaged signals.The invention is therefore capable of application in distancemeasurement for a plurality of sets of laser signals.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to theskilled in the art). Therefore, the scope of the appended claims shouldbe accorded to the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

1. A distance measurement method for use in a laser range finding deviceto measure a distance between the laser range finding device and atarget, comprising: transmitting a laser signal to the target at a firsttime point; receiving a reflected laser signal reflected by the target;generating a digital signal by sampling the reflected laser signal andcreating a plurality of different non-zero magnitude values for thedigital signal that are each associated with a time for the receivedreflected laser signal and have a magnitude that corresponds to amagnitude of the received reflected laser signal at the associated time;and calculating the distance between the laser range finding device andthe target based on the generated digital signal.
 2. The method of claim1, wherein: the digital signal is generated based on one reflected lasersignal.
 3. The distance measurement method of claim 1, wherein: thedigital signal is a signal with eight bits per data value.
 4. Thedistance measurement method of claim 1, wherein: the digital signal is asignal with more than one bit per data value.
 5. The distancemeasurement method of claim 1, wherein: the digital signal is generatedby sampling the reflected laser signal with a sampling signal.
 6. Thedistance measurement method of clam 1, wherein: the generated digitalsignal has a waveform that is similar to a waveform of the receivedreflected laser signal.
 7. A laser range finding device for measuring adistance between the laser range finding device and a target,comprising: a transmitter that transmits a laser signal to the target ata first time point; a receiver that receives a reflected laser signalreflected by the target; an analog-to-digital converter that samples thereflected laser signal and generates a digital signal by sampling thereceived reflected laser signal and creating a plurality of differentnon-zero magnitude values that are each associated with a time for thereceived reflected signal and have a magnitude that corresponds to amagnitude of the received reflected laser signal at the associated timesuch that the generated digital signal has a waveform that is similar toa waveform of the received reflected laser signal; and a processing unitthat calculates the distance between the laser range finding device andthe target based on the generated digital signal.
 8. The laser rangefinding device of claim 7, wherein the transmitter comprises: a laserdiode; and a driving circuit that controls the laser diode to transmitthe laser signal to the target.
 9. The laser range finding device ofclaim 7, wherein the receiver comprises: an avalanche photodiode thatreceives the reflected laser signal reflected by the target; a signalamplifier that amplifies the reflected laser signal received by theavalanche photodiode; and a filter circuit that performs a noisefiltering to the amplified reflected laser signal.
 10. The laser rangefinding device of claim 7, further comprising: a memory unit incommunication with the processing unit, the memory unit stores thedigital signal; and a control circuit in communication with theprocessing unit, the control circuit receives a trigger signal from theprocessing unit so as to drive the transmitter to transmit the lasersignal and activate the analog-to-digital converter.
 11. The laser rangefinding device of claim 7, further comprising: a displaying unit coupledto the processing unit for displaying the distance between the laserrange finding device and the target calculated by the processing unit.12. The laser range finding device of claim 14, wherein: the digitalsignal is a signal with more than one bit per data value.
 13. The laserrange finding device of claim 7, wherein: the digital signal is a signalwith eight bits per magnitude for the plurality of different non-zeromagnitude values.
 14. The laser range finding device of claim 7,wherein: the analog-to-digital converter samples the reflected lasersignal with a sampling signal to generate the digital signal.
 15. Thelaser range finding device of claim 7, wherein: the analog-to-digitalconverter generates the digital signal based on one reflected lasersignal.
 16. A distance measurement method for use in a laser rangefinding device to measure a distance between the laser range findingdevice and a target, comprising: transmitting a laser signal to thetarget at a first time point; receiving a reflected laser signalreflected by the target; generating a digital signal having a pluralityof signal values by sampling the reflected laser signal, the signalvalues range from 0 to N where N is a positive integer greater than two;and calculating the distance between the laser range finding device andthe target based on the generated digital signal.
 17. The method ofclaim 16, wherein: the digital signal is generated based on onereflected laser signal.
 18. The distance measurement method of claim 16,wherein: the digital signal is a signal with more than one bit per datavalue.
 19. The distance measurement method of claim 16, wherein: thedigital signal is generated by sampling the reflected laser signal witha sampling signal.
 20. The distance measurement method of claim 16,wherein: the generated digital signal has a waveform that is similar toa waveform of the received reflected laser signal.